It is a common requirement of medical services that use blood bags that the seals through which access is to be had to the bag for the purpose of using the contents of the bag shall be tamper-proof. That is to say once the seal has been broken it either is incapable of reclosing or else there is an obvious signal that it has been broken. This ensures that when the contents of the bag are used it can be ascertained that they have not been adulterated, infected or substituted.
It is customary never to withdraw all the blood or blood products from the bag since it is a routine precaution to retain some in the bag for analysis in the event that anything untoward happens to the patient. There might be a suspicion of mis-matching of blood groups, for example. For this purpose some bags are equipped with, as well as the tamper-proof initial seal, a replaceable cap which prevents leakage of the remaining content or air access to the remaining contents and which is placed over the access orifice at which the tamper-proof seal was originally present. These caps have usually been readily removable and the situation could be envisaged of fraudulent tampering with the remaining contents of the bag if it was known that an enquiry was likely; or of inadvertent removal of the cap with contamination or loss of those remaining contents.